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Fish Assemblage Dynamics in the Tagus and Sado Estuaries (Portugal)

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Fish Assemblage Dynamics in the Tagus and Sado Estuaries (Portugal) Cah. Biol. Mar. (2008) 49 : 23-35 Fish assemblage dynamics in the Tagus and Sado estuaries (Portugal) Ana NEVES1, Henrique CABRAL1, Ivone FIGUEIREDO2, Vera SEQUEIRA1, Teresa MOURA2 and Leonel Serrano GORDO1 (1) Faculdade de Ciências da Universidade de Lisboa, Departamento de Biologia Animal & Instituto de Oceanografia, Bloco C2, Campo Grande, 1749-016 Lisboa, Portugal Fax: +351 217500028. E-mail: [email protected] (2) Departamento de Recursos Marinhos, IPIMAR, Av. Brasília, 1449-006 Lisboa, Portugal Abstract: From May 2001 to April 2002 the ichthyofauna from Tagus and Sado estuary was monthly sampled. 25 species from 18 families were identified in Tagus samples, and 38 species from 21 families in Sado estuary. Only 14 species (26%) occurred in the two estuaries, but 14 families (58%) were common for both. Pomatoschistus minutus, Gobius niger, Syngnathus sp and Sardina pilchardus were the dominant species in Tagus estuary. Gobius niger, Monochirus hispidus, Halobatrachus didactylus, Arnoglossus laterna and Diplodus bellottii were the dominant species in Sado. From CCA analysis a separation of Tagus samples from those of Sado is evident. Furthermore, in Sado estuary a spatial pattern is evident, while in Tagus estuary no pattern is evident among sampling areas or seasons. The physical factors which have a major influence are salinity, total organic mater and percentage of mud, being the last two highly related. Comparing the diversity index in the two estuaries the values are higher in Sado estuary probably due to a greater influence of seawater and a higher variability of sediment, which induce to a more rich community. Résumé : Dynamique des assemblages de poissons dans les estuaires du Tage et du Sado (Portugal). La faune ichthyologique des estuaires du Tage et du Sado a été échantillonnée mensuellement depuis le mois de mai 2001 jusqu’au mois d’avril 2002. On a identifié 25 espèces de 18 familles dans le Tage et 38 espèces de 21 familles dans le Sado. Seules 14 espèces (26%) sont communes aux deux estuaires ; 14 familles (58%) sont communes. Les espèces dominantes du Tage sont Pomatoschistus minutus, Gobius niger, Syngnathus sp et Sardina pilchardus et dans le Sado se détachent Gobius niger, Monochirus hispidus, Halobatrachus didactylus, Arnoglossus laterna et Diplodus bellottii. L’analyse canonique des correspondances a montré une séparation nette entre les échantillons du Tage et ceux du Sado. En outre, une structure spatiale est visible dans l’estuaire du Sado, alors qu’aucune structure, ni spatiale ni saisonnière, n’apparaît pour le Tage. Les facteurs physiques les plus influents sont la salinité, la matière organique totale et le pourcentage de vase, ces deux derniers se trouvant fortement corrélés. Keywords: Fish assemblage l Environmental factors l Estuarine system l Tagus l Sado l Portugal Reçu le 2 mai 2007 ; accepté après révision le 11 mars 2008. Received 2 May 2007; accepted in revised form 11 March 2008. 24 FISH ASSEMBLAGES IN TWO ESTUARIES Introduction Sado estuary has an area of approximately 180 km2 of which 44% is intertidal (Vasconcelos et al., 2007). The river Estuaries have long been known to be important as flow regime presents significant interannual and seasonal nurseries areas, migration routes and habitats for estuarine variations and is highly dependent on the amount of rain residents (Haedrich, 1983). They are also very important occurring in the south part of Portugal, varying from 1 m3 for human activities, and often associated to large urban or s-1 in summer to 60 m3 s-1 in winter (Martins et al., 2001). industrial areas. Along with anthropogenic pressures (e.g. Only 5 small tributaries provide freshwater to the estuary poldering and land-claim, channel management, power and, consequently, the estuary has a high salinity, above stations and water abstraction, barrages) (Cattrijsse et al., 29.5, all over the year (Cunha, 1994). The tidal regime in 2002), estuaries are affected by natural changes in the area is primarily semidiurnal with a range of 0.6 to1.5 environmental condition. Temperature and salinity m in neap tides and of 1.6 to 3.5 m in spring tides (Martins gradients, alterations of water-column turbidity and in et al., 2001; Amaral & Cabral, 2004). chemical composition including changes in nutrients, dissolved gases and trace metals are, among others factors, Sampling procedures prominent features of estuarine systems likely to affect fish From May 2001 to April 2002, 4 sites in the Tagus and 5 populations (Elliott & McLusky, 2002). sites in the Sado estuary were sampled monthly (Fig. 1), Fish have very diverse live history strategies and occupy using a 4-m beam trawl with 10-mm mesh size. This gear is a variety of ecological niches using the estuary in several one of the more appropriate to the estuarine environment different forms. Some species have obligate live history (Potts & Reay, 1987) due to its reduced selectivity when stages in the estuary such as estuarine resident or compared to other gears. Trawls were towed for 15 min at diadromous fishes, others are facultative users (marine or 1.85 km h-1, during daylight and low water of spring tides. freshwater opportunists) and others only occasionally find After capture, specimens were kept at low temperature in their way into an estuary (marine vagrants) (Able, 2005). an icebox and later identified, measured to the nearest mm The rivers Tagus and Sado form two of the major (total length, TL) and weighed to the nearest 0.01 g (wet estuarine systems in the Atlantic coast of the Iberian weight, WW) in the laboratory. The identification was Peninsula (Costa, 1988; Cunha, 1994). Both estuaries conducted according to Whitehead et al. (1986). suffer the influences of industrial complexes, farming, During all samplings, temperature (± 0.1ºC), salinity (± urban areas and fishing (Cabral, 1998), activities that are 0.1) and dissolved oxygen concentration, DO (± 0.1 mg l-1) often linked with alterations of the health status and were measured using a multiparameter water quality probe ecological integrity of estuaries. Over two million people (YSI, model 85). In spring (May), summer (August), live in the area of influence of the Tagus estuary and over autumn (November) and winter (February), sediment 300 000 people live around Sado estuary. samples were collected using a van Veen grab and analysed Because estuaries are water-bodies sensitive to the for granulometric purposes. These samples were washed to adverse effects of human impacts (Elliott & McLusky, remove the fine portion (< 0.063 mm), dried at 70ºC and 2002) it is of major importance to investigate the nature of then sorted in a 1-mm sieve to separate gravel from sand. estuarine communities to aid preservation and management The organic matter in the sediment was also determined by of these areas (Elliott, 2002). the loss of weight on ignition at 450ºC for two hours. The aim of the present study is two-fold: 1) to identify Species were grouped into ecological guilds according to the main patterns of spatial and temporal variation in the common traits of life history, biology and/or behaviour (see fish assemblage of each estuary, and 2) to describe the Elliott & Dewailly, 1995). environmental conditions associated with the fish biota. Data analysis Material and Methods For data analysis, months were grouped in seasons: March, April, May (spring); June, July, August (summer); Study areas September, October, November (autumn); December, Tagus estuary has an area of 325 km2 of which 40% is January, February (winter). k-dominance curves intertidal area (Cabral, 1998) and a tidal range of 4.6 m (Lambshead et al., 1983) were used to graphically account with a mean depth less than 10 m (França et al., 2005). The for diversity outline. For each sampling site, the species mean Tagus river flow is 400 m3 s-1, being highly variable richness (Margalef, 1958), the Shannon-Wiener diversity both seasonally and interannually (França et al., 2005). index (Shannon & Weaver, 1949) and the evenness index Beyond the Tagus, 15 tributaries contribute to the import of (Pielou, 1977) were used to characterize fish temporal and freshwater into the estuary. spatial abundance. Significant differences on the abiotic A. NEVES, H. CABRAL, I. FIGUEIREDO, V. SEQUEIRA, T. MOURA, L. S. GORDO 25 Figure 1. Map of the sampling sites in Tagus and Sado estuaries. Figure 1. Localisation des points d’échantillonnage aux estuaires du Tage et du Sado. parameters and ecological indices were evaluated among Results sites and seasons (Kruskal-Wallis test) and estuaries (Mann-Whitney test) for a significant level (α) of 0.05 (Zar, Abiotic parameters 1999). Fish (excluding the species with abundance lower than Mean values of temperature and dissolved oxygen were not 1%) and environmental data matrix were submitted to significantly different between estuaries (U = 0.242, p = canonical correspondence analysis (CCA) in order to obtain 0.809, U = 1.691, p = 0.091 respectively). Mean salinity a direct environmental interpretation of the extracted values were significantly higher (always above 30) in Sado ordination axes (Ter Braak, 1994). Despite the fact that estuary (U = -8.7888, p < 0.001), while in Tagus estuary the CCA is a linear technique, it is less susceptible than other values varied between 10.3 and 28.3 (Fig. 2). ordination methods to non-linearities in the data matrix, Seasonally, Tagus estuary presented significant because it explicitly assumes non-linear responses of differences in temperature and salinity (Table 1) varying the species along environmental gradients (McGarigal et al., former from a mean value of 13ºC in winter to 22ºC in 2000). CCA was performed using CANOCO 4.5 (Ter Braak summer. The latter varied from a mean value of 17 in spring & Smilauer, 1998). to 24 in summer. Sado estuary presented significant 26 FISH ASSEMBLAGES IN TWO ESTUARIES Table 1.
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